Method for the production of a semiconductor substrate comprising a plurality of gate stacks on a semiconductor substrate, and corresponding semiconductor structure

ABSTRACT

Method for the production of a semiconductor structure comprising a plurality of gate stacks on a semiconductor substrate which serve as control electrodes for a respective selection transistor of a corresponding memory cell comprising a storage capacitor. Gate stacks are provided next to one another on the substrate provided with a gate dielectric wherein the gate stacks have a lower first layer made of polysilicon, an overlying second layer made of metal silicide, and an upper layer made of silicon nitride. A sidewall oxide is formed on uncovered sidewalls of the first and second layers of the gate stacks, and at least partly the sidewall oxide is removed on those sidewalls of the gate stacks serving as a control electrode which are remote from the associated storage capacitor. Silicon nitride sidewall spacers are then formed on the gate stacks.

The present invention relates to a method for the production of asemiconductor structure comprising a plurality of gate stacks on asemiconductor substrate, and to a corresponding semiconductor structure.

U.S. Pat. No. 6,080,613 discloses a semiconductor structure comprising aplurality of gate stacks, the gate stacks having thinned sidewallspacers made of silicon oxide.

U.S. Pat. No. 5,439,835 discloses the use of a mask for forming sidewallspacers on gate stacks.

U.S. Pat. No. 6,383,863 B1 discloses gate stacks comprising caps made ofsilicon nitrite.

Although applicable in principle to any desired integrated circuits, thepresent invention and the problem area on which it is based areexplained with regard to integrated DRAM memory circuits in silicontechnology.

The memory cells of integrated DRAM memory circuits comprise a bit linecontact, a selection transistor and a storage capacitor. The capacitormay be embodied as a trench capacitor or as a stacked capacitor abovethe gate lines.

The bit line contacts are etched in a self-aligning manner between gatelines arranged in strip-type fashion and are subsequently filled with aconductive material, metal or highly doped polysilicon. In specificdesigns, a bit line contact is used for two adjacent memory cells whichlikewise has a space-saving effect.

As memory cell dimensions decrease, the lateral opening of the bit linecontacts also becomes smaller and smaller since the distance between thegate lines decreases. Therefore, considerable efforts have to be made inorder to keep the resistance of the bit line contact plug small, whichis achieved for example by changing from a polysilicon filling to asuitable metal filling (e.g.) tungsten).

It is also a central problem to find a suitable etching for the bit linecontact, that is to say an etching of doped SiO₂ selectively withrespect to the silicon nitride covering the gate lines, the etchingensuring an opening of the bit line contact hole without damaging thegate lines or the insulation thereof. This is critical in particularowing to the process and thickness fluctuations occurring in the layersthat form or insulate the gate lines.

What are particularly critical are the width of the gate line stacks,the thickness of the side wall spacer and also the thickness of a linermade of silicon nitride that functions as a diffusion barrier in orderto avoid impurities of the doped SiO₂ in the gate lines.

In the case of the 110 nm technology, by way of example, taking accountof these process and thickness fluctuations, a fluctuation range of 45nm to 75 nm remains for the width of a respective bit line contact.

Considerable efforts have been made heretofore in order, by way ofexample, to optimize the aforementioned self-aligned contact holeetching of the doped SiO₂ for the bit line contacts. A principaldifficulty consists in the fact that the lateral dimensions are reducedby the shrink factor but the height of the gate stack is scarcelyreduced. This increasingly worsens the so-called aspect ratio(height/width) for the critical contact hole etching.

Nowadays the etching process is realized by a two-stage etching. In thefirst step, etching is effected as far as possible perpendicularlyanisotropically as far as the silicon nitride cap, and, in the secondstep, etching is effected as far as possible selectively with respect tothe silicon nitride cap, in which case the profile of the upper regionof the contact hole should as far as possible not be expanded.

Reducing the material thicknesses for the individual constituent partsof the gate line stack likewise requires considerable efforts. By way ofexample, it is known that even only slightly reducing the thickness ofthe side wall oxide considerably impairs the leakage current behavior ofthe selection transistors, with the result that the required retentiontime of the memory signal in the storage capacitor can no longer beguaranteed.

The problem area on which the present invention is based consists inalleviating the space problem for the bit line contacts.

According to the invention, this problem is solved by means of theproduction method specified in claim 1 and the correspondingsemiconductor structure according to claim 9.

The particular advantages of the production method according to theinvention and of the corresponding semiconductor structure are that thealleviated space conditions mean that it is possible to achieve areduction of the short-circuit problem area during the contact holeetching, with the result that the yield in the process, for example inthe DRAM fabrication process, can be significantly increased.Furthermore, the invention affords the possibility of further shrinks.

The idea on which the present invention is based consists in increasingthe lateral width for the bit line contact by single-sided reduction ofthe sidewall oxide on the side of the bit line contact, that is to sayon that side of the selection transistor which is remote from thestorage capacitor of the respective memory cell. In other words, thesidewall oxide on the selection transistor is configured asymmetrically,that is to say thicker on the side of the storage capacitor and thinneron the side of the bit line contact.

For this purpose, the sidewall oxide on the bit line side of theselection transistor may be merely thinned or completely removed, as aresult of which the lateral opening for the bit line contact can beenlarged by at most twice the width of the sidewall oxide.

In the 110 nm technology, by way of example, the thickness of thesidewall oxide amounts to 2×12 nm=24 nm. That is to say that given alateral width of the bit line contact of 45 nm to 75 nm, the maximumlateral space gain corresponds to 53% to 32% of the current lateralwidth of the bit line contact, which means a considerable alleviationfor the self-aligned contact hole etching.

In this case, the sidewall oxide on the selection transistor on the sideof the storage capacitor is left unchanged by virtue of a suitablemasking, which has the effect that it is possible to avoid an impairmentof the leakage current behavior of the memory cell.

Advantageous developments and improvements of the respective subjectmatter of the invention are found in the subclaims.

In accordance with one preferred development, the gate stacks areprovided in parallel strip-type fashion on the semiconductor substrate.

In accordance with a further preferred development, the gate stacks areprovided with approximately the same configuration and approximately thesame spacing on the semiconductor substrate.

In accordance with a further preferred development, the active regionsof the selection transistors are provided in a self-aligning manner inthe interspace between the gate stacks.

In accordance with a further preferred development, the removal of thesidewall oxide on the sidewalls of the gate stacks serving as a controlelectrode takes place using a mask.

In accordance with a further preferred development, the sidewall oxideis removed on opposite sidewalls of adjacent gate stacks aboverespective active regions which simultaneously serve as a connection fortwo selection transistors.

In accordance with a further preferred development, a respective bitline contact is formed between sidewalls of adjacent gate stacks with anat least partly removed sidewall oxide.

In accordance with a further preferred development, the gate stacks arecreated by carrying out an application and patterning of the first, theoverlying second and the upper third layer on the gate dielectric.

In accordance with a further preferred development, the third layer is asilicon nitride layer.

An exemplary embodiment of the invention is illustrated in the drawings,and explained in more detail in the description below.

FIGS. 1-5 show diagrammatic illustrations of successive method stages ofthe method for the production of a semiconductor structure comprising aplurality of gate stacks on a semiconductor substrate as an embodimentof the present invention.

In FIGS. 1-5, identical reference symbols designate identical orfunctionally identical constituent parts.

In FIG. 1, reference symbol 1 designates a silicon semiconductorsubstrate, in which trench capacitors TK1, TK2, TK3, TK4 forcorresponding memory cells are provided. Reference symbols 60 to 65designate doping regions introduced into the semiconductor substrate 1,which are drain and source regions of the selection transistors shown inthis section, which are driven by the gate stacks GS2, GS3, GS6, GS7.The regions 60, 62, 63, 65 simultaneously serve as connection regionsfor the associated storage capacitors TK1-TK4. The regions 61 and 64 arein each case simultaneously associated with two adjacent selectiontransistors.

The gate stacks GS1, GS4, GS5, GS8 are associated with selectiontransistors in other rows of the memory cell matrix. The thin gate oxidedesignated by reference symbol 5 in the figures lies beneath the gatestacks GS1-GS8 on the semiconductor substrate 1. The gate stacks GS1 toGS8 are provided in strip-type fashion on the semiconductor substrate 1and all have the same construction and also approximately the same sizeand the same spacing from one another.

In the process stage illustrated in FIG. 1, the gas stacks GS1 to GS8have a lower layer 10 made of polysilicon and an overlying layer 20 madeof tungsten silicide. A cap 30 made of silicon nitride is provided abovethat. The respective left-hand and right-hand sidewalls of thestrip-type gate stacks GS1 to GS8 are uncovered in this stage.

The active regions 60 to 65 between the gate stacks GS1 to GS7 canexpediently be introduced by self-aligned implantations between the gatestacks GS1 to GS8.

In the process step illustrated in FIG. 2, the two sidewalls of the gatestacks GS1 to GS8 are oxidized in order to form a customary sidewalloxide 40 having a thickness of typically 4-15 nm. This sidewall oxide 40provides for electrical insulation for preventing short circuits betweenbit line and gate line.

In the subsequent process step illustrated in FIG. 3, a mask M isapplied to this structure, which mask is patterned in such a way thatthose sidewalls of the gate stacks which are remote from the trenchcapacitors are uncovered, which are associated in this section with theselection transistors for the relevant storage capacitors. They are thegate stacks GS2, GS3, GS7 and GS6 in the present case. Corresponding bitline contacts are later to be created between the gate stacks GS2, GS3and GS7, GS6, respectively. In this section, no connection for a laterbit line contact is situated between the gate stacks GS4, GS5 and on theleft next to the gate stack GS1, and on the right next to the gate stackGS8, so that masking by the mask M is not necessary here but it wouldnot, in principle, be a disturbance either.

In the next process step shown in FIG. 4, the sidewall oxide 40 isthinned on the non-masked uncovered sidewalls of the gate stacks GS1,GS2, GS3, GS4, GS5, GS6, GS7, GS8, which leaves a sidewall oxide 40′thinned by approximately 50% on the relevant sidewalls.

This thinning of the sidewall oxide may be effected using a wet-chemicaletching with a suitable oxide-etching solution, e.g. HF, diluted HF(DHF), buffered HF (BHF) or buffered oxide etch (BOE), or by means of adry etching method, such as e.g. a reactive ion etching method.

In the course of the thinning process, the gate oxide present at thebottom between the relevant gate stacks is also reduced or removed,which does not have a disturbing effect.

It should be noted that in these regions between the gate stacks GS2,GS3 and between the gate stacks GS6, GS7 where the bit line contactshave to be produced later, more lateral space is available later as aresult of the sidewall oxide being thinned on both sides.

With reference to FIG. 5, finally, in a known manner, a respectivesidewall spacer 50 made of silicon nitride is provided for the purposeof protecting the sidewall oxide 40 and the thinned sidewall oxide 40′during the later formation of the bit line contacts between the gatestacks GS2, GS3 and GS6, GS7 where, as a result of the thinning, thereis a larger lateral width d′ present in comparison with otherintermediate regions without thinning.

The further steps for completing the memory cells are known in the priorart and, therefore, are not illustrated further here. In particular, asstandard, firstly a liner made of silicon nitride is applied as adiffusion barrier, then a doped layer made of SiO₂ is deposited andplanarized, and, finally, the bit line contact holes are etched in aself-aligned manner between the gate stacks GS2, GS3 and GS6, GS7 in theabovementioned two-stage etching process and the bit line contact isfilled with a suitable metal, such as e.g. tungsten.

On account of the fact that the sidewall oxide is only reduced on theside remote from the storage capacitor, the leakage current behavior ofthe memory cells remains unchanged.

Although the present invention has been described above on the basis ofa preferred exemplary embodiment, it is not restricted thereto, butrather can be modified in diverse ways.

In particular, the selection of the layer materials for the gate stacksand the arrangement thereof are only by way of example and can be variedin many different ways.

List of reference symbols

-   1 Substrate-   GS1-GS8 Gate stack-   10 Polysilicon-   20 Silicide-   30 Silicon nitride-   40, 40′ Sidewall oxide-   5 Gate dielectric-   M Mask-   60-65 Active regions-   50 Sidewall spacer

1. A method for the production of a semiconductor structure comprising aplurality of gate stacks on a semiconductor substrate, which serve ascontrol electrodes for a respective selection transistor of acorresponding memory cell comprising a storage capacitor having thesteps of: a) providing the gate stacks next to one another on thesemiconductor substrate provided with a gate dielectric, the gate stackshaving a lower first layer made of polysilicon, an overlying secondlayer made of a metal silicide, in particular a tungsten silicide, andan upper third layer made of silicon nitride; b) forming a sidewalloxide on uncovered sidewalls of the first and second layers of the gatestacks; c) removing at least partly the sidewall oxide on thosesidewalls of the gate stacks serving as a control electrode which areremote from the associated storage capacitor; and e) forming siliconnitride sidewall spacers on the gate stacks.
 2. The method as claimed inclaim 1, wherein the gate stacks are provided in parallel strip-typefashion on the semiconductor substrate.
 3. The method as claimed inclaim 2, wherein the gate stacks are provided with approximately thesame configuration and approximately the same spacing on thesemiconductor substrate.
 4. The method as claimed in claim 3, whereinactive regions of the selection transistors are provided in aself-aligning manner in the interspace between the gate stacks.
 5. Themethod as claimed in claim 1, wherein the removal of the sidewall oxideon the sidewalls of the gate stacks serving as a control electrode takesplace using a mask.
 6. The method as claimed in claim 1, wherein thesidewall oxide is removed on opposite sidewalls of adjacent gate stacksabove respective active regions which simultaneously serve as aconnection for two selection transistors.
 7. The method as claimed inclaim 1, wherein a respective bit line contact is formed betweensidewalls of adjacent gate stacks with an at least partly removedsidewall oxide.
 8. The method as claimed in claim 1, wherein the gatestacks are created by carrying out an application and patterning of thefirst, the overlying second and the upper third layer on the gatedielectric.
 9. A semiconductor structure comprising a plurality of gatestacks on a semiconductor substrate provided with a gate dielectric,which serve as control electrodes for a respective selection transistorof a corresponding memory cell comprising a storage capacitor, the gatestacks having a lower first layer made of polysilicon, an overlyingsecond layer made of a metal silicide, in particular a tungstensilicide, and an upper third layer made of silicon nitride, and having asidewall oxide on the sidewalls of the first and second layers of thegate stacks, the sidewall oxide being thinned or removed on thosesidewalls of the gate stacks serving as a control electrode which areremote from the associated storage capacitor, and the gate stacks havingsilicon nitride sidewall spacers.
 10. The method as claimed in claim 9,wherein the third layer is a silicon nitride layer.
 11. The method asclaimed in claim 1, wherein the silicon nitride sidewall spacers areformed on the sidewalls on the gate stacks.
 12. A semiconductorstructure comprising a plurality of gate stacks on a semiconductorsubstrate provided with a gate dielectric, which serve as controlelectrodes for a respective selection transistor of a correspondingmemory cell comprising a storage capacitor, the sidewall oxide beingthinned or removed on those sidewalls of the gate stacks serving as acontrol electrode which are remote from the associated storagecapacitor.